1. Field of the Invention
The present invention relates to an image-sensing device that yields an output logarithmically proportional to the amount of incident light, and to a signal processing method used in such an image-sensing device.
2. Description of the Prior Art
A conventional area sensor having photosensitive devices such as photodiodes outputs a signal that is linearly proportional to the brightness of the light incident on the photosensitive devices as shown at (a) in FIG. 8. When a subject having brightness distribution as shown at (b) in FIG. 8 is shot with such a linear-conversion-based area sensor (hereafter referred to as a xe2x80x9clinear sensorxe2x80x9d), no brightness data is obtained outside the roughly two-digit brightness range (dynamic range) within which the linear sensor can effectively perform image sensing (this brightness range will be referred to as the shootable brightness range). (Here, the brightness range is represented as Lmax/Lmin, assuming that the brightness of the subject distributes from a minimum value Lmin [cd/m2] to a maximum value Lmax [cd/m2], and the shootable brightness range is called the dynamic range).
This is because the output range is proportional to the dynamic range, and is thus narrow. Accordingly, when an image is reproduced on an output device, such as a display, that uses a signal obtained from such a linear sensor, the reproduced image suffers, outside the shootable brightness range, flat blackness in a low-brightness region and flat whiteness (saturation) in a high-brightness region. To avoid such flat blackness or flat whiteness, it is necessary, according as the brightness distribution, shown at (b) in FIG. 8, of the subject varies, to shift the shootable brightness range of the area sensor by varying the aperture value and shutter speed of a camera, or by varying the integration time of the incident light.
On the other hand, the assignee of the present invention once proposed an area sensor (hereafter referred to as a xe2x80x9cLOG sensorxe2x80x9d) provided with a photosensitive means that produces a photoelectric current in accordance with the amount of incident light, a MOS transistor to which the photoelectric current is fed, and a bias means for biasing the MOS transistor in such a way that a subthreshold current flows therethrough, so that the photoelectric current is converted logarithmically (refer to U.S. Pat. No. 5,241,575). As shown at (a) in FIG. 9, a LOG sensor like this yields an output whose level is natural-logarithmically proportional to the brightness of incident light, and thus offers as wide as a five- to six-digit dynamic range. This permits the brightness distribution, shown at (b) in FIG. 9, of a subject to lie within the shootable brightness range in most cases even if the brightness distribution varies.
An area sensor such as a linear sensor or LOG sensor as described above is usually incorporated in an image-sensing device formed on a one-chip IC (integrated circuit) together with circuits that perform various kinds of signal processing, such as HVC (hue-value-chroma) processing performed to convert the RGB signals from the area sensor into HVC signals representing hue, value, and chroma respectively, color conversion performed to convert signals to achieve proper color matching and thereby achieve proper color rendering on an output device such as a display or printer, and edge enhancement performed to enhance sharpness in edge portions of a reproduced image. Most image-sensing devices are designed for use with a linear sensor, and therefore the circuits provided therein as the stage succeeding the area sensor to perform various kinds of signal processing, such as HVC processing, color conversion, and edge enhancement, are designed to process a signal obtained from a linear sensor.
For this reason, when a LOG sensor, which offers a wide dynamic range, is incorporated as an area sensor in an image-sensing device, the circuits provided therein as the stage succeeding the area sensor to perform various kinds of signal processing need to be modified so as to be able to process a signal obtained from, not a conventionally commonly used linear sensor, but a LOG sensor.
An object of the present invention is to provide an image-sensing device that incorporates a LOG sensor offering a wide dynamic range but that nevertheless permits the output from the LOG sensor to be processed properly even if the circuits provided therein as the stage succeeding the LOG sensor to perform various kinds of signal processing are left unmodified from circuit designs configured to process a linearly converted signal.
To achieve the above object, according to one aspect of the present invention, an image-sensing device is provided with: a photoelectric converter, having a photosensitive device that produces an electric signal in accordance with the amount of incident light, for logarithmically converting the electric signal produced by the photosensitive device and then outputting the converted electrical signal; a signal processing circuit; and an output converter for converting the electric signal output from the photoelectric converter into an electric signal linearly proportional to the amount of incident light and for feeding to the signal processing circuit the thus converted electrical signal which is linearly proportional to the amount of incident light.